Use generic applications as default for certain mime types
Fixes #608.
I used the xfce-mimeapps.list file to be as unobtrusive as possible, since this file is only used under XFCE and at a lower priority than any other default applications file.
I did have to add to the original desktop files the mime types as standard keys rather than XFCE extensions so that GIO would recognize them.
https://www.freedesktop.org/wiki/Specifications/mime-apps-spec/